Method of and apparatus for controlling plural fluid medium thermal power plants



Dec. 3, 1968 1 HELLER ET AL 3,413,805

METHOD OF AND APPARATUS FOR CONTROLLING PLURAL FLUID MEDIUM THERMAL POWER PLANTS Filed March 22, 1966 nitecl States Patent 3,413,805 METHOD OF AND APPARATUS FOR CON- TROLLING PLURAL FLUID MEDIUM THERMAL POWER PLANTS Laszl Heller and Lszlo Forgo, Budapest, Hungary, as-

signors to Komplex Nagyberendezesek Export-Import Vallalata, Budapest, Hungary, a firm Filed Mar. 22, 1966, Ser. No. 538,896 Claims priority, application Hungary, Mar. 29, 1965, HE456 4 Claims. (Cl. 60-38) ABSTRACT OF THE DISCLOSURE Method and apparatus for controlling a plural m dium power plant in which the heat content of a working medium working between higher temperature limits (steam) is transmitted through a heat exchanger to a working medium working between lower temperature limits by continuously maintaining the minimum pressure of the steam higher than atmospheric pressure.

This invention relates to a method of and apparatus for controlling plural fluid medium thermal power plants.

Thermal power plants are already known Which are 0perated with two different fluid mediums in such a manner that steam is employed as working medium of an upper stage and a so-called cold vapour such as ammonia is used as working medium of a lower stage. Such thermal power plants are, as a rule, distinguished by two advantages. First, their use permits the building of power plants of very high unit capacities by eliminating the restrictions on power capacity due to the volume increase of steam which is extraordinarily great with high vacuum conditions. Viz., the saturation pressure of the so-called cold vapours associated with normal ambient temperatures is by orders of magnitude higher than that of steam and, thus, also the volume of vapour withdrawing from a turbine is by orders of magnitude smaller, identical outputs being taken for the purpose of comparison. Th second basic advantage of the aforesaid power plants is that in case of using air for recooling purposes it permits the full use of possible cooler air temperatures. Moreover, such systems offer the advantage of eliminating the use of vacuum in the whole system if a suitably selected cold vapour is employed, While, namely, with a merely steam operated turbine it is practically impossible to obviate vacuum in the course of condensation, the saturation pressure of most of the cold media is even at the practically occurring lowest temperature levels higher than the atmospheric pressure. Thereby, it is rendered possible that, if steam is condensated in heat exchangers of the system at a pressure higher than the atmospheric value, vacuum may completely be eliminated from the entire system. Then, operation of the system is substantially simplified and, at the same time, also some of the apparatus may be dispensed with. Above all, no vacuum tight apparatus and vacuum generating units such as vacuum air pumps, water pumps, coolers, etc., otherwise indispensable with systems of the conventional type are necessary any more. Moreover, heat transmission conditions are not deteriorated by the presence of air on heat transmitting surfaces, i.e. in the heat exchanger between steam and cold vapour and in the cold vapour condensers. Thus, the use of such systems is advantageous with respect to both apparatus requirements and operational conditions.

Thus, operation can substantially be simplified while, however, special requirements have to be met with as regards controlling. By suitably selecting the cold vapour, no vacuum will occur in the cold vapour condenser, Likewise, by suitable designing it is possible to ensure that the outlet pressure of the steam turbine exceeds the atmospheric value of 1 absolute atmosphere when the system is operated under intended conditions. At the same time, for economical reasons, the lower limit f the steam cycle will preferably be selected as low as possible since the efficiency of a steam cycle is, practically, always superior to that of cold vapour cycles. Therefore, the plant has to be seized so that the outlet pressure of the steam exceed the atmospheric value only by a possibly small difierence. This, in turn, requires a controlling system which operates to prevent vacuum on the steam side with decreasing loads. Viz., with conventional controlling systems where the amount of steam entering the steam turbine is controlled by impulses normally taken from a speed governor, with decreasing loads the pres sures in a heat exchanger between steam and cold vapour downstream the steam turbine will be smaller on both sides of the heat exchanger surface in correspondence with a new condition of equilibrium, in consequence whereof the automatically adjusted pressure prevailing on the steam side can drop below 1 atmosphere. The regulation has to prevent also in such load cases that vacuum appear in the system.

The main object of the present invention is the provision of a control method by which the maximum pressure prevailing on the cold vapour side is influenced in such a manner that in the heat exchanger for steam and cold vapour in correspondence to the actual power requirements a temperature difference be present by which it is ensured that the temperature on the steam side always surpasses a temperature of degrees centigrade which means that the pressure prevailing on the steam side can never drop below the atmospheric value.

Accordingly, the invention is primarily concerned with a method of controlling a plural medium thermal power plant with which the heat content of a working medium for working between higher temperature limits is transmitted through a heat exchanger to a working medium working between lower temperature limits, and the main feature of the invention consists in that, in addition to a power control conventionally employed with heat engines, the minimum pressure of steam used as working medium between said higher temperature limits is kept so as to be always higher than the atmospheric pressure.

Further features and objects of the invention will be described by taking reference to the accompanying drawings which show, by way of example, heat flow diagrams of two embodiments constructed and operated in compliance therewith. Which of the illustrated embodiments will actually be employed depends on. the temperature difference selected in correspondence with further viewpoints.

The exemplified embodiment shown in FIG. 1 applies to cases where, according to the result of economical calculations carried out in consideration of other viewpoints, the temperature difference in the steam-cold vapour heat exchanger will not be greater than 5 to 10 degrees centigrade. In such case, a load governor 1 permits an amount of steam to enter a steam turbine 2 which amount of steam corresponds to the power requirements of the case. This amount of steam flows from the steam turbine 2 through a pipe conduit 3 into a steam-cold vapour heat exchanger 4 where it becomes condensed. The condensate resulting from such condensation is supplied by a pump 5 back into a steam boiler not shown. At the same time, pump 6 for cold vapour such as ammonia supplies liquid ammonia into the heat exchanger 4 between steam and ammonia where the ammonia evaporates and flows through a pipe conduit 7 into a liquid trap 8. Herefrom, the ammonia vapour flows through a pipe conduit 9 and a pressure controller valve as well as through a pipe conduit 19 into an ammonia turbine 11 where it expands to condenser pressure and meanwhile, drives an electric generator 15 having a common shaft 14 with the steam turbine 2. Thereafter, the ammonia flows through a pipe conduit 12 into an ammonia condenser 13. Obviously, separate shafts for both turbines 2 and 11 might be employed as well. Moreover, the steam turbine 2 may be associated with a separately driven generator.

The liquid ammonia is pumped by a pump 16 from the condenser 13 through a preheater 17, furthermore through a pipe conduit 18 into the liquid trap 8. Thus, besides the load governer 1 the system comprises only one further controlling means which is the pressure regulator valve 10 and which operates in such a manner that the amount of ammonia vapour permitted to enter the ammonia vapour turbine is adjusted so that a prescribed constant pressure be maintained in the pipe conduit 9 upstream thereof which means practically the ammonia side of the steam-ammonia heat exchanger 4.

Should the load governor 1, on decreasing load, admit less steam into the turbine 2, the (outlet) pressure behind the turbine 2 would correspondingly decrease and so would the pressure of ammonia vapour on the other side of the heat exchanger 4. Upon the effect of decreasing pressure, the regulator 10 ahead of the ammonia turbine 11 permits less steam to flow toward the ammonia turbine 11 so as to maintain the pressure prevailing in the ammonia vapour space ahead of the regulator 10. This will prevent also the pressure on the steam side of the heat exchanger 4 from dropping below a value so determined that it should always be above the atmospheric.

By way of example:

Let us assume a condensing temperature of steam of 110 C. at full load (with the corresponding saturation pressure in condensation), and an evaporation temperature of ammonia of 104 C. (with the corresponding pressure of ammonia saturation on the other side of the surface). Should now the load decrease to percent and the load governor 1 ahead of the steam turbine should correspondingly let less steam pass, then the regulator 10 ahead of the ammonia turbine will let a correspondingly smaller quantity of steam pass, so as to keep pressure, naturally tending to decrease, at the preset value. The characteristics of this regulator 10 should be determined in such a way that with full steam flow it should keep ammonia pressure ahead of the regulator 10 at the saturation pressure that corresponds to 104 C. saturation temperature and that at the same time, with a quantity of vapour corresponding to 40 percent load, keep this same pressure, so that, as a saturation pressure, it should correspond to 99 C. saturation temperature. In this case the condensing temperature on the steam side will drop e.g. from 110 to 102 C. temperature.

(Although this decrease would correspond to a percent temperature drop only, considering that simultaneously, with smaller temperature difference, the heat transfer coefficient on the ammonia side deteriorates considerably, the 50 percent may also continue to decrease to, let us say, 40 percent.)

Should, as a result of the optimum calculation, a temperature difference in the heat exchanger, in excess of 5 to 10, seem more economical, then another regulation method would have to be chosen. With a greater temperature difference it would not be advisable (according to the above regulation) to increase the pressure of steam; it would rather be preferable to create a larger temperature gap by determining a lower pressure for the ammonia vapour. In such a case, on the other hand, the control system should be so designed as to maintain the steam pressure and not the pressure of the ammonia vapour above a certain minimum. This essentially means that the dimensions of the heat transfer area must be varied as a function of the changes in load. Two basic possibilities exist to achieve this:

In the first alternative the effective surface is so diminished that the removal of condensate arising on the steam side of the heat exchanger 4 is slowed down so that the rising condensate should flood the appropriate portion of the cooling surface, cutting it out in this way from the heat transfer. Here the drawback is that the insertion of a separate water storage tank is needed and that the heat exchanger 4 itself fills up with water and increases thereby very considerably in weight. This fact in View of the vast dimensions required for very large outputsmay raise serious problems. At the same time. the control of the condensate pump may also impose a difficult task since, instead of the conventional level control, some other, obviously more complicated, solution must be found.

The scheme according to FIGURE 2 presents a simpler solution to the problem. The pressure of ammonia vapour is left freely to adjust to the load, but the pressure on the steam side is maintained at a predetermined lowest level. To achieve this the ammonia side of the heat exchanger surface is divided into several parallel sections 20 whose inlet valves 21 are operated by a pressure regulator controlled from the steam side as indicated at 22 in such a way that, upon decreasing steam-side pressure, the valves 21 consecutively close while, upon the effect of rising steam-side pressure, they consecutively open. Thereby, under the lowest rate of load, only as large an evaporating surface remains active as ensures the temperature difference required to keep the pressure on the steam side above atmospheric, i.e. temperature on this side remains at all times above C.

Any one of the control systems here described will, by simple means, ensure that pressure relations, even under the smallest possible load acting on the machine. should not cause vacuum to arise at any point of the system.

What we claim is:

1. A method of controlling a plural medium thermal power plant wherein the heat content of a working medium working between higher temperature limits in a first portion of said plant including, in part, a heat exchanger, is transmitted through said heat exchanger to a working medium working between lower temperature limits in a second portion of said plant including, in part. said heat exchanger, said working medium working between higher temperature limits being steam, which comprises detecting changes in pressure in said working medium working between higher temperautre limits; and continuously maintaining the minimum pressure of said working medium working between higher temperature limits higher than atmospheric pressure in response to said detected change of pressure by adjusting said second portion of said plant.

2. Apparatus for controlling a plural medium thermal power plant including a working medium working between higher temperature limits, said medium being steam, a working medium working between lower temperature limits, and a heat exchanger for transmitting the heat content of said working medium working between higher temperature limits to said working medium working between lower temperature limits comprising, an automatic pressure controlling means on the side of the heat exchanger associated with said working medium working between lower temperature limits adapted to keep the pressure of said working medium working between lower temperature limits at a predetermined value, whereby the minimum pressure of said working medium working between higher temperature limits is maintained higher than atmospheric pressure.

3. Apparatus for controlling a plural medium thermal power plant as recited in claim 2, including a pressure feeler disposed at the inlet for said working medium working between higher temperature limits of said heat exchanger for detecting the pressure of said working medium working between higher temperature limits, said pressure feeler being operatively connected to said automatic pressure controlling means.

4. Apparatus for controlling a plural medium thermal power plant including a working medium working be tween higher temperature limits, said medium being steam, a working medium working between lower temperature limits, and a heat exchanger for transmitting the heat content of said working medium working between higher temperature limits to said working medium working between lower temperature limits, said heat exchanger having an inlet for receiving said working medium working between higher temperature limits comprising a pressure feeler located at said heat exchanger inlet for detecting the pressure of said working medium working between higher temperature limits, said heat exchanger having a plurality of parallel branches for conducting said working medium working between lower temperature limits; and a plurality of automatic closing devices operated sequentially by said pressure feeler, one of said automatic closing devices being located in each of said heat exchanger branches, whereby the minimum pressure of said working medium working between higher temperature limits is maintained higher than atmospheric pressure.

References Cited UNITED STATES PATENTS 3,257,806 6/1966 Stahl 6036 107,206 9/ 1870 Morley 6038 1,167,158 1/1916 Emmet 60-38 1,632,575 6/1927 Abendroth 60-38 1,982,745 12/ 1934 Koenemann 6038 FOREIGN PATENTS 261,368 11/1926 Great Britain.

398,533 9/1933 Great Britain.

460,466 1/1937 Great Britain.

369,923 2/ 1923 Germany.

480,522 8/ 1929 Germany.

20 MARTIN P. SCHWADRON, Primary Examiner.

R. R. BUNEVICH, Assistant Examiner. 

